Nanoparticulate materials have received much attention recently as a result of their unique physical properties and applications in a variety of devices and products. Nanoparticulate silica and alumina colloids have been known for many years and have many uses in industrial, medical and consumer products. Much effort has been given to the development of novel nanoparticulates, and to the study of their physical properties. A nanoparticle has dimensions on the order of a molecular scale, typically between about 1-100 nm, or 10−9 to 10−7 meters.
Layered compounds are a unique class of materials that have strong chemical bonding in two-dimensions but only weak interactions in the third-dimension. As a result, layered compounds often display unique chemical and physical properties such as the ability to adsorb or intercalate ions, compounds and organic molecules. Laboratory experiments have demonstrated the use of layered compounds in the controlled release of functional chemistry, in the transport of biological materials, and in drug-delivery. Layered compounds are employed commercially in the design of novel composites, as additives in health and beauty products, as barrier layers in polymeric systems, as ion-exchange materials, rheological modifiers, to name only a few applications. Clays are an important class of layered compounds which find many commercial applications. Examples of clays include the minerals bentonite, montmorillonite, hectorite and the synthetic clay, laponite. These are alumino-silicate based materials whose structure consists of stacks of alumino-silicate sheets separated by cations, such as Na+, K+ and Ca2+. The cations may be exchanged for other cations, such as metal-ions, or by cationic organic molecules such as quaternary ammonium compounds, (e.g., CH3(CH2)nN+R3). Clays of this nature are therefore often referred to as “cationic clays”. Cationic clays are commercially available as nanoparticulates under the tradename Laponite, and find many applications in various articles.
Layered double hydroxides, closely related to the mineral “hydrotalcite”, are a unique class of layered compounds. These materials share layered structural characteristics with their cousin “cationic clays”, but rather than cations, layered double hydroxides contain anions between their metal hydroxide sheets. For this reason they are often referred to as “anionic clays”. The interlayer anions contained within layered double hydroxides may be exchanged for other anions (eg., Cl−, Br−, I−, NO3−, CO32−, SO3−) and by anionic organic compounds and especially organic compounds containing anionic funtional groups (carboxylates, sulfonates, and phosphates). Thus, layered double hydroxides are uniquely suited to develop complex inorganic-organic hybrids with anionically charged organic molecules. Anionically charged organic molecules and supra-molecules (e.g., DNA) are particularly prevalent in biological systems and thus layered double hydroxides are well suitable for forming hybrids with bio-molecules.
The synthesis of layered double hydroxides has been reviewed by a number of authors, and several synthetic approaches have been disclosed. See, e.g., W. T. Reichle, “Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite) Solid State Ionics, 22, 135-141 (1986); F. Cavani, F. Trifiro and A. Vaccari, “Hydrotalcite-type anionic clays: preparation, properties and applications”, Catalysis Today, 11, 173-301 (1991); F. Trifiro and A. Vaccari, “Hydrotalcite-like anionic clays (Layered double hydroxides), in Comprehensive Supramolecular Chemistry, “Solid State Supramolecular Chemistry: Two- and Three-dimensional Inorganic Networks”, Alberti G.; Bein, T. Eds., Elsevier, N.Y., Chapter 8 (1996). By far the most commonly used method involves the coprecipitation of an aqueous solution of mixed-metal salts through the addition of a base. In the coprecipitation method, the precipitation has been described as being accomplished by (a) slow (dropwise) addition of a mixed-metal solution to a basic solution; (b) addition of a basic solution to a vigorously stirred solution of the mixed-metal salts, or (c) slow (dropwise) addition of a mixed-metal solution to a basic solution at a constant pH. Other methods of producing layered double hydroxides involve hydrothermal synthesis, and the so-called “reconstruction method”. The hydrothermal methods are not commonly used since they are generally more difficult to carry out and produce products having a large particle size (typically greater than 1-2 microns). The reconstruction method is useful for carrying out ion-exchange reactions of hydrotalcite, but is not used to prepare pristine layered double hydroxides.
U.S. Pat. No. 6,329,515 B1 to Choy et al. describes bio-inorganic hybrid composites which are able to retain and carry bio-materials with reversible dissociativity. The composites comprise layered double hydroxides having intercalated therein a bio-material. The invention also describes methods of preparation of the composites which comprises coprecipitating, with an alkaline material, an aqueous solution comprising an bivalent metal (M(II)) and trivalent metal (M(III)) at a specified molar ratio. The particles size and colloidal stability of the resulting product, however, are not reported.
Layered double hydroxide “bio-nanohybrids” are discussed by Choy et al. in J. Mater. Chem. 11, 1671(2001). However, the particles size and colloidal stability of the resulting product are not determined, and no evidence is provided for composites having nanoscale dimensions. The term “nanohybrid” is apparently used to describe the interlayer dimensions of the intercalated biocomposites (i.e, the distance separating two adjacent layers) and not the particle dimensions of the pristine layered double hydroxide, or the resulting intercalated layered double hydroxide.
WO 03/011233 A1 to Choy et al. describes hybrid materials comprised of an active component for raw materials for cosmetics and a layered metal hydroxide. The application describes materials and methods for preparing hydrozincytes (Zn5(OH)82 anion), and electron micrographs contained therein show some particles apparently having nano-sized dimensions. The particle size distributions of the materials, however, are not reported. The application also describes methods for surface modification of the particles to obtain dispersions with improved colloidally stability. Layered double hydroxides having nano-sized dimensions, and colloidally stable layered double hydroxide dispersions, however, are not demonstrated. The surface modification methods employed further are difficult and expensive to perform, and may disrupt the ability of the layered materials to act as chemical delivery agents.
There remains a need for anionic clays having nanoparticulate dimensions and for stable colloidal dispersions of anionic clays. There remains a need for colloidally stable, nanoparticle dispersions of anionic clays having a high percent solids. There remains a need for colloidally stable, nanoparticle dispersions of anionic clays which do not contain surface modification reagents such as polymers, surfactants, silicates or organo-silanes. There is a need for methods of preparing colloidally stable, nanoparticle dispersions of anionic clays which are efficient and provide the product at an industrial scale and at a low-cost.